This invention relates to crosslinked polymers derived from the polymerization of unsaturated polyoxyethylene monomers and to soft contact lenses prepared from said polymers.
Soft hydrogel contact lenses are currently the lens design of choice for extended wear applications. These lenses are derived from the polymerization of a hydrophilic monomer such as hydroxyethyl methacrylate (HEMA).
A contact lens composed of the polymerization reaction product of HEMA (polyHEMA) is swollen in water to prepare a hydrogel. For conventional higher water-containing hydrogels the water content of the hydrogel lens is an important factor in patient comfort because the permeability of oxygen through the lens is proportional to its water content. Since the cornea of a contact lens wearer needs oxygen for metabolism, the water content of the lens, and hence its oxygen permeability, are important factors in achieving an acceptable degree of wearer comfort and corneal health.
Although polyHEMA lenses can be swollen with water to prepare hydrogels with minimally acceptable water contents and oxygen permeability, lenses composed of polyHEMA alone do not have adequate mechanical properties for routine handling and care. Therefore, commercially available contact lenses contain not only HEMA, but also a crosslinking monomer to enhance the mechanical properties of the finished lens. The crosslinking monomer usually used is ethylene glycol dimethacrylate (EGDMA). While the crosslinking monomer improves the mechanical properties of the finished lens, and therefore enhances the handleability of the lens, it also has adverse consequences. Conventional crosslinking agents reduce the water content of the finished lens and increase its brittleness. The reduced water content lowers the permeability of oxygen through the lens, which in turn decreases patient comfort over an extended period of wear. The increase in brittleness of the lens makes the lens more fragile, and therefore more susceptible to tearing.
Since neither polyHEMA alone nor the reaction product of HEMA with a crosslinking agent has produced optimum properties for a soft contact lens, commercially available lenses typically incorporate additional monomeric components from which the lens is derived. For example, anionic monomers such as methacrylic acid (MAA) are added to increase the water content of the lens, and hydrophobic monomers such as alkyl (meth)acrylates, are added to enhance mechanical properties of the lens. However, there is still room to improve the properties of soft hydrogel contact lenses.
Many disclosures are found in the literature of attempts to fabricate hydrogel contact lenses from unique polymer systems. The following is a representative list of such disclosures:
U.S. Pat. No. 3,988,274 PA1 U.S. Pat. No. 5,034,461 PA1 U.S. Pat. No. 4,780,487 PA1 U.S. Pat. No. 4,780,488 PA1 European Patent Application 321,403 PA1 U.S. Pat. No. 4,921,956 PA1 U.S. Pat. No. 5,196,458 PA1 European Patent Application 394,496 PA1 European Patent Application 493,320 PA1 (i) a momomer of Formula (II) wherein n is a number having a value so that the diunsaturated polyoxyethylene monomer has a molecular weight within the range of from about to about 1700, and X, m and R.sup.1 are as defined above in connection with the monounsaturated polyoxyethylene monomer; PA1 (ii) a monomer of the formula: ##STR1## wherein R.sup.1, m and X are as defined above, and p+q are selected so that the monomer represented by Formula (III) has a molecular weight within the range of from about 500 to 1900; and PA1 (iii) mixtures of monomers of Formulas (II) and (III), having the molecular weights defined in this paragraph (C); and
While numerous attempts have been made to optimize the properties of soft contact lenses, these attempts have fallen short of the goal of fabricating lenses with not only properties suited for patient comfort during extended wear, but also outstanding mechanical properties. What is needed is a polymer that has the requisite properties to achieve the highest degree of patient comfort without appreciably sacrificing its mechanical properties when the polymer is fabricated into a soft hydrogel contact lens.